nix-gc: check for nix.enable

This was added to Nixpkgs in eb8b70c020e6693b29634660fa173d7f14f882eb.

(cherry picked from commit fc9367a9ec)

modules/services/nix-gc/default.nix

@@ -62,15 +62,20 @@ in
 
   ###### implementation
 
-  config = mkIf cfg.automatic {
+  config = {
+    assertions = [
+      {
+        assertion = cfg.automatic -> config.nix.enable;
+        message = ''nix.gc.automatic requires nix.enable'';
+      }
+    ];
 
-    launchd.daemons.nix-gc = {
+    launchd.daemons.nix-gc = mkIf cfg.automatic {
       command = "${config.nix.package}/bin/nix-collect-garbage ${cfg.options}";
       environment.NIX_REMOTE = optionalString config.nix.useDaemon "daemon";
       serviceConfig.RunAtLoad = false;
       serviceConfig.StartCalendarInterval = cfg.interval;
       serviceConfig.UserName = cfg.user;
     };
-
   };
 }